Optical communication devices with miniaturized size, reduced cost and multiple functions have been researched and developed. As one example, silica waveguides formed on silicon substrate have been researched. Several methods for fabricating the silica waveguides, such as flame hydrolysis deposition (FHD), low pressure chemical vapor deposition (LP-CVD) and plasma chemical vapor deposition (P-CVD), have been proposed. However, these methods have the problem that film fabricated thereby may include a relatively large strain. A solution to this problem is reported in T. Shimoda et al., "Low Temperature Process of Silica Waveguides by TEOS-O.sub.2 Atmospheric Pressure CVD", Proceedings of The 1996 IEICE General Conference, C-230 (1996), where atmospheric pressure CVD(AP-CVD) that is conventionally used to form insulating film in semiconductor process is employed to provide SiO.sub.2 film with a reduced strain.
However, such an AP-CVD method has the problem that the throughput must be lowered as SiO.sub.2 micro-particles to be generated by vapor-phase reaction are very slowly deposited on the substrate. Therefore, this method is not suitable for mass production and the product obtained becomes thereby costly. Furthermore, the fabricating equipment is also costly as the AP-CVD method requires a severe control in vapor-phase reaction.